Cast iron alloy



United States Patent CAST IRON ALLOY Coolidge A. Karns, Canton, Ohio, assignor to United Engineering & Foundry Company, Pittsburgh, Pa., a corporation of Pennsylvania 'No Drawing. Application August 1, 1952, Serial N0. 302,253

12 Claims. (Cl. 75128) This invention relates to alloy cast irons. The cast irons of this invention are useful for various 'purposes. However, their properties are such as to make them especially useful for rolling mill rolls such, for example, as the working rolls of the finishing stands of hot strip mills, and in cold mills. For that reason the invention will be described with particular reference to rolls, although not by way of limitation.

Rolling mill rolls have been Widely made during recent years from martensitic alloy cast irons containing about 3.25 per cent of carbon, about 1.5 per cent of chromium, about 1 per cent of manganese, about 4.5 per cent of nickel, and about 1 per cent of silicon. Such rolls possess an average Shore hardness of about 78 to 83, and at that hardness an average impact strength of about 45 foot pounds, maximum average transverse strength of about 4000 pounds per square inch (p. s. i.), and an average maximum deflection of about 0.065". Those rolls have been used so widely that they afford a good standard for judging other cast irons, and they will be referred to hereinafter as the comparison rolls or cast irons. Despite the fact that those rolls perform satisfactorily, it would be desirable to have alloy cast irons of higher physical properties and of better wear resistance.

A primary object of the invention is to provide alloy cast irons adapted particularly, but not exclusively, to the production of rolling mill rolls and which are characterized by exceptionally high impact and transverse strengths at high surface hardnesses.

A further object is to provide alloy cast irons in accordance with the foregoing object and which likewise possess higher deflection values and higher hardness than other alloy cast irons known to me and used for making cast iron rolls.

Yet another object is to provide alloy cast irons that combine high deflection, exceptionally high impact and transverse strengths with high hardness that is of unusual uniformity across the surface of rolls and other castings made from them.

Still another object is to provide alloy cast irons'in accordance with any or all of the foregoing objects that may be chill or sand cast, and the microstructure of which may be varied according to specific needs of the casting, and castings of which possess exceptionally good wear, or abrasion resistance.

A still further object is to provide cast iron rolls made from alloy cast irons in accordance with any or all of the foregoing objects.

I have discovered, and it is upon this that the invention is in large part predicated, that the objects of the invention are supplied by alloy cast irons having substantially greater amounts of chromium and silicon than are in the comparison rolls identified above. The irons of this invention comprise, as essential alloying elements, about 2.5 to 4 per cent of carbon, from about 5 to about 8 per cent of chromium, about 0.4 to l per cent of manganese, about 4 to 6.5 per cent of nickel, more than about 2 and up to about 3.5 per cent of silicon. Desirably, these cast 2,762,704 Patented Sept. 11, 1956 irons contain molybdenum also, say up to about 1.5 per for if they fall below the lower limits stated the hardness and transverse strength that characterize these new irons become unsatisfactorily low.

Phosphorus and sulfur should not exceed about 0.2 and 0.1 per cent respectively. The remainder of the cast II'OHS of this invention consists essentially of iron although there may be present smaill amounts of other elements such as are common in cast irons of this. general type, provided they do not detrimentally affect the properties that characterize the alloys of this invention.

At about 5 per cent or more chromium the castings exhibit an essentially martensitic eutectic structure characterized by the substantial absence of graphite. This is particularly desirable and is preferred, because the hardness of the martensite matrix approaches that of the cementite more nearly than in the case of the aforesaid comparison rolls. Consequently, these new rolls have excellent wear resistance due to more nearly uniform hardness across the casting, or roll, face.

This preferred eutectic structure is obtained by appropriate relationship of the alloying elements. I. e., as the content of chromium is increased the amount of silicon may be reduced, and likewise carbon may be decreased as chromium is increased. All three of those elements tend to produce an eutectic structure. Generally, as the carbon is increased the amount of nickel can be lowered, and vice versa. Also, as chromium is increased the contents of carbon and silicon should be increased if high strength is desired. For small rolls nickel may be used in the lower portion of the range stated, and in the higher portion of the range for larger rolls.

In the preferred embodiment of the invention the irons contain from 6.5 to 7 per cent of chromium, about 5.5 per cent of nickel, 0.5 to 0.6 per cent of molybdenum, 3.10 to 3.20 per cent of carbon, more than 2 to about 2.5 per cent of silicon, and about 0.55 per cent of manganese.

Microhardness measurements across the surfaces of eutectic chill castings made from my new alloy irons have.

shown that the hardness is more uniform across the roll face than is the case with similarcastings made from the aforesaid comparison cast iron. In other words, the hardness of the martensite matrix is closer to that of the cementite than is the case in the comparison cast irons. This is evidenced by the following characteristic data:

Vickers Penetration This characteristic is of major importance in improving Wear resistance, and particularly in the case of rolls for the finishing stands of cold mills because it means that in continued use the surface finish of rolled sheet is more perfect as a result of reduced differential wear between the martensite matrix and the cementite in the roll surface. 7

Another major characteristic of castings made in accordance with this invention is that when chill cast they exhibit average Shore hardnesses of to or higher whereas the average hardness of the comparison rolls ranges from 78 to 83. Of particular importance is the fact that the transverse strength and deflection of castings inaccordance with the invention are much higher than in the case of the greater hardness of the former. test bars of my new irons show comparison irons despite the to 0.15 inch, and higher, and transverse strengths as high as 7100 p. s. i.; and in all cases those properties average substantially higher than those of the comparison irons.

Furthermore, the impact strength of 1 /4 inch test bars of the alloy irons of this invention are exceptionally high, for example, from 80 to 90 foot pounds (Izod; 1 A unnotched bar). In some instances 1% inch test bars made from the irons of this invention have not been broken at 120 foot pounds although the hardness was from 85 Shore or higher.

As an example, reference may be made to castings made from irons according to the invention containing from 3.3 to 3.4 per cent of carbon, from to 5.5 per cent of chromium, about 5.3 per cent of nickel, and 2.25 per cent of silicon, with manganese, phosphorus and sulfur as stated above. The structure was largely untempered martcnsite, together with cementite and some austenite. The surface hardness of the castings was 86 to 87 Shore, and the transverse strength of 1% inch bars averaged 6000 to 7000 p. s. i.

As another example, 1% inch diameter test bars of an iron containing 3.14 per cent of carbon, 4.96 per cent of chromium, 0.43 per cent of manganese, 0.5 per cent of molybdenum, 4.4 per cent of nickel, and 3.25 per cent of silicon, with phosphorus and sulfur below 0.1 per cent, possessed an as cast transverse strength of 5360 to 5780 p. s. i., deflection of 0.115 to 0.12, impact strength of 75 to 85 foot pounds, and Rockwell C hardness of 55.5, equivalent to 76 to 77 Shore. The deflection and impact strengths are much higher than are produced by the comparison iron referred to above.

As further evidencing the exceptional impact strength at high hardness of the alloy irons of this invention, a ball Weighing 17,000 pounds repeatedly dropped freely through 20.5 feet failed to break a chill cast roll 22" in diameter and 47" long made from iron in accordance with this invention.

Preferably my alloy irons are chill cast, which develops a fine condensed grain chilled surface. When so cast the Shore hardness does not drop more than one point up to a depth of about 1 /2 from the surface, which is desirable from the standpoint of roll users because it permits repeated dressing of the roll surface before it must be retired from service.

However, these new irons may likewise be sand cast, in which condition they possess the important characteristics described. For instance, slinger liners made from the alloys of this invention last in use from 24 to 30 hours as compared to 12 hours for similar liners made from the comparison iron described above and which in turn exhibit less life than liners made from other cast irons and tool steels.

Although it is preferred for many purposes that products in accordance with the invention be used in the as cast condition, they may be heat treated if desired for particular purposes.

This application is a continuation-in-part of my copending application filed June 4, 1951, Serial No. 231,655, now abandoned which was a continuation-in-part of my application Serial No. 165,371, filed May 31, 1951, now abandoned.

According to the provisions of the patent statutes, 1 have explained the principle of my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, with in the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. A martensitic alloy cast iron comprising from about 2.5 to 4 per cent of carbon, from about 5 to 8 per cent of chromium, from about 4 to 6.5 per cent of nickel, more than 2 to 3.5 per cent of silicon, and the remainder substantially all iron.

. 2. A martensitic alloy cast iron comprising from about 2.5 to 4 per cent of carbon, from about 5 to 8 per cent of chromium, about 0.4 to 1 per cent of manganese, molybdenum up to 1.5 per cent, from 4 to 6.5 per cent of nickel, more than 2 to about 3.5 per cent of silicon, and the remainder substantially all iron.

3. Cast iron according to claim 2 containing not over about 0.2 per cent of phosphorus and not over about 0.1 per cent of sulfur.

4. A martensitic alloy cast iron comprising from about 3.1 to 3.2 per cent of carbon, from 6.5 to 7 per cent of chromium, about 0.5 to 0.6 per cent of molybdenum, 5.5 per cent of nickel, more than 2 to 2.5 per cent of silicon, about 0.55 per cent of manganese, not over about 0.1 per cent each of phosphorus and sulfur, and the remainder substantially all iron.

5. A martensitic chilled alloy cast iron roll comprising from about 2.5 to 4 per cent of carbon, from 5 to 8 per cent of chromium, from about 4 to 6.5 per cent of nickel, more than 2 to about 3.5 per cent of silicon, and the remainder substantially all iron, and the roll having a fine condensed grain surface chill.

6. A martensitic chilled alloy cast iron roll comprising from about 2.5 to 4 per cent of carbon, from 5 to 8 per cent of chromium, about 0.4 to 1 per cent of manganese, molybdenum up to about 1.5 per cent, from about 4 to 6.5 per cent of nickel, more than 2 to 3.5 percent of silicon, and the remainder substantially all iron, and the roll having a fine condensed grain surface chill.

7. A roll according to claim 6, phosphorus being not over about 0.2 per cent, and sulfur not over about 0.1 per cent.

8. A martensitic chilled alloy cast iron roll comprising about 3.1 to 3.2 per cent of carbon, about 6.5 to 7 per cent of chromium, about 0.55 per cent of manganese, about 0.5 to 0.6 per cent of molybdenum, about 5.5 per cent of nickel, more than 2 to about 2.5 per cent of silicon, and the remainder substantially all iron, and the roll having a fine condensed grain surface chill.

9. An iron casting having a section at least 1% thick formed of alloy cast iron comprising from about 2.5 to 4% of carbon, from about 5 to 8% of chromium, from about 4 to 6.5% of nickel, more than 2 to 3.5% of silicon, and the remainder substantially all iron, and character ized by a fine grained and densely martensitic eutectic structure the hardness of which approaches that of cementite dispersed therethrough.

10. An iron casting in accordance with claim 9 in which the carbon is about 3.1 to 3.4%, the chromium about 6.5 to 7%, the nickel about 5.5%, the silicon more than 2 to 2.5%.

11. A chilled alloy cast iron roll comprising from about 2.5 to 4% of carbon, from about 5 to 8% of chromium, from about 4 to 6.5% of nickel, more than 2 to about 3.5% of silicon, and the remainder substantially of iron, and the roll having a fine condensed grain sur-- face chill the hardness of which is substantially uniform to a depth of about 1.5" from the surface, and the roll being characterized by a fine grained, densely martensitic structure the hardness of which approaches that of cementite dispersed therethrough.

12. A roll according to claim 11 in which the carbon is about 3.1 to 3.4%, the chromium about 6.5 to 7%, the nickel about 5.5%, and the silicon more than 2 to about 2.5%.

References Cited in the file of this patent UNITED STATES PATENTS 1,626,248 Merica et al. Apr. 26, 1927 2,189,131 Cape et al Feb. 6, 1940 2,662,011 Gagnebin et al. Dec. 8, 1953 

9. AN IRON CASTING HAVING A SECTION AT LEAST 1 1/4" THICK FORMED OF ALLOY CAST IRON COMPRISING FROM ABOUT 2.5 TO 4% OF CARBON, FROM ABOUT 5 TO 8% OF CHROMIUM, FROM ABOUT 4 TO 6.5 OF NICKEL, MORE THAN 2 TO 3.5% OF SILICON, AND THE REMAINDER SUBSTANTIALLY ALL IRON, AND CHARACTERIZED BY A FINE GRAINED AND DENSELY MARTENSITIC EUTECTIC STRUCTURE THE HARDNESS OF WHICH APPROACHES THAT OF CEMENTITE DISPERSED THERETHROUGH. 